Previous friction clutches consist of a multiplicity of small parts in order to achieve relatively high torque that is smooth in output. This creates a number of problems. Using a large number of parts increases manufacturing cost and inherent inaccuracies due to tolerance stack-up. The chances for parts not working together properly is also increased. Resulting problems can include inconsistent torque, backlash from poorly fitted parts; and with more parts, increased chances for failure. Also, it is time consuming and costly in terms of labor to assemble a clutch with a variety of small pieces. The present high cost of labor makes this type of clutch less desirable to produce now than in the past.
While a variety of inventions exist, many have involved complex designs that do not lend themselves to vary low-cost manufacturing and assembly procedures. For example, U.S. Pat. No. 3,712,438 is a clutch with radially expanding friction surfaces. Specifically, this is a centrifugally operated clutch. This type of clutch engage when rotational speeds increase sufficiently to force the inner friction surface into the outer surface.
Such clutch designs do not produce torque unless the inner member is rotating. Once a critical speed is reached, the inner expanding hub and driven member rotate in unison. As a result, this type of design is not made for slipping between members except for the brief period while engagement occurs.
Wrap spring type clutches are commonly used when slipping between friction surfaces is required. As the friction surfaces wear however, the spring must wrap down further to maintain constant torque. Torque output is very sensitive to wear due to the limited amount of radial movement of the wrap spring that is possible. Wear of 0.002-0.005 inches in the friction surfaces will cause the torque to drop off by a relatively large amount. This creates a significant limitation to clutch life. To reduce wear, expensive coatings or surface hardening of the friction surface must be done.
Furthermore, uneven radial forces result when the spring winds down onto the hub. This causes uneven wearing of friction surfaces. The full area of the friction element cannot be utilized to its optimum potential. Additional manufacturing complexities and costs are required to overcome this problem. U.S. Pat. No. 3,405,929 shows one attempt to reduce uneven wear of the friction surfaces. Wrap spring clutches typically have only one friction surface. With one friction element, torque can be increased only by increasing pressure between friction surfaces. Once maximum allowable surface pressure between friction surfaces has been reached, torque can be increased only by adding additional friction surfaces. U.S. Pat. No. 3,242,696 shows two friction surfaces. This increases the complexity of the design. With two concentric rings required to support friction surfaces, the chances for misalignment are greater.
Adding a second outer friction surface necessitates reducing the operating diameter of the first friction surface. Since torque is a function of force and distance from the center of rotation, reducing the diameter of the first friction surface reduces the torque it can produce. It also limits heat dissipation since heat is generated further away from the outer surface where it can be dissipated. Increasing the friction surfaces to three or more would compound the previously stated problem even more.
U.S. Pat. Nos. 5,037,354 and U.S. Pat. No. 5,092,440 are typical small clutches of simple design. Both designs require springs and additional supporting parts to urge the friction elements together. This increases cost and complexity. Both designs show only one friction surface. As described with wrap spring clutches, providing only one friction surface limits the maximum amount of torque which can be produced. Increasing the number of friction surfaces beyond the one shown increases cost and complexity still further. If it is even possible at all. The low torque output limits commercial applications since many and users require relatively high torque.
U.S. Pat. No. 4,878,880 shows a simple two-piece clutch. Both parts can be made as a molded plastic or metal part reducing costs further. There are at least two major shortcomings. First, the slipping elements jump from radial groove to groove as the clutch is rotated. This is described in column 5 lines 2 through 6 of U.S. Pat. No. 4,878,880. As a result, torque is not smooth. For current usage in business machines, for example; smooth torque is a prime concern. Second, torque is produced by cantilevered fingers extending axially from a plate. As the distance from the mounting plate increases, a cantilevered beam will have lower resistance to deflection. Therefore the outward radial force exerted along the length of the fingers is not consistent. Wear will be inconsistent since the part of the finger closest to the plate will not deflect as easily. By the very nature of the design, the full length of the friction elements cannot be fully utilized. As a result, clutch life and torque output will be reduced.